The present invention relates to a method for reclaiming a delaminated wafer produced as a by-product in the so-called ion implantation and delamination method in which a bonded wafer such as SOI (silicon on insulator) wafer is produced by bonding an ion-implanted wafer to another wafer and then delaminating them, and a reclaimed wafer.
As conventional methods for producing wafers of an SOI structure, there are the SIMOX (separation by implanted oxygen) method in which oxygen ions are implanted into a silicon single crystal at a high concentration and then the crystal is subjected to a heat treatment at high temperature to form an oxide film, and a bonding method in which two of mirror polished wafers are bonded without using an adhesive and one of the wafers is made into a thin film.
However, as a method for producing an SOI wafer, the method comprising bonding a wafer implanted with ions and then delaminating the wafer to produce an SOI wafer (a technique called ion implantation and delamination method: Smart Cut Method (registered trademark)) is recently coming to attract much attention. This method is a technique for producing an SOI wafer, wherein an oxide film is formed on at least one of two silicon wafers, either hydrogen ions or rare gas ions are implanted into one wafer from its top surface to form a micro bubble layer (enclosed layer) within this silicon wafer, then the ion-implanted surface of the wafer is brought into contact with the other silicon wafer via the oxide film, thereafter the wafers were subjected to a heat treatment to delaminate one of the wafer as a thin film at the micro bubble layer as a cleavage plane, and the other wafer is further subjected to a heat treatment to obtain an SOI wafer having firm bonding (refer to Japanese Patent Laid-open (Kokai) Publication No. 5-211128). In this method, the cleaved surface is obtained as a good mirror surface, and SOI wafers also showing high uniformity of SOI layer thickness are obtained relatively easily.
More recently, there is also developed another technique, although it is a kind of the ion implantation and delamination method, in which the delamination can be performed at room temperature without performing any particular heat treatment by exciting hydrogen ions to be implanted and implanting them in a plasma state.
According to these ion implantation and delamination methods, silicon wafers can also be directly bonded to each other without using an oxide film after the ion implantation, and they can be used not only for a case where silicon wafers are bonded to each other, but also for a case where a silicon wafer is implanted with ions and directly bonded to an insulator wafer composed of SiO2, SiC, Al2O3 or the like to form an SOI layer. Further, if a wafer other than a silicon wafer (SiO2, SiC, Al2O3 etc.) is used as a wafer to be implanted with ions, it is also possible to obtain a bonded wafer having a thin film composed of such a material.
When a bonded wafer such as SOI wafer is produced by such ion implantation and delamination method, one delaminated wafer of silicon should be inevitably produced as a by-product. Conventionally, it has been considered that, since one SOI wafer can be obtained from substantially one silicon wafer by reclaiming such a delaminated wafer produced as a by-product in the ion implantation and delamination method, the cost can be markedly reduced.
However, such a delaminated wafer is not one that can be used as it is as a usual mirror surface silicon wafer, and it may have a step on peripheral portions, have a damage layer on the delaminated plane due to the ion implantation, or show significant surface roughness. Therefore, in order to reclaim it as a mirror surface wafer, it is necessary to polish (reclaim polishing) the surface to remove the step and the damage layer and thereby improve the surface roughness.
However, inventors of the present invention found that, even if the surface roughness of a delaminated wafer is improved by performing polishing as described above, there remained the following problem. FIG. 4 includes schematic views for explaining the problem.
As shown in FIG. 4A, a chamfered portion 8 is usually provided by a processing called chamfering at a peripheral portion of a bond wafer 2 to be implanted with hydrogen ions in order to prevent cracking, chipping and so forth during processing. As for this bond wafer, an oxide film 3 is formed on its surface by performing thermal oxidation treatment as required as shown in FIG. 4B.
Then, when hydrogen ions are implanted from the top surface of such a bond wafer 2, a micro bubble layer 4 is formed in parallel to the wafer top surface (a portion from the wafer top surface to the micro bubble layer 4 formed by the implanted ions will be referred to as an ion-implanted layer hereinafter), and the ion-implanted layer 9 is also formed on the chamfered portion 8 as shown in FIG. 4C.
The bond wafer 2 on which the ion-implanted layer 9 was formed as described above is brought into close contact with a base wafer via the oxide film 3, and then subjected to a heat treatment so that they should be bonded. Then, they are separated into an SOI wafer and a delaminated wafer 5. At this time, the ion-implanted layer 9 on the chamfered portion 8, which was not bonded to the surface of base wafer, would remain on the delaminated wafer 5 even after the delamination as shown in FIG. 4D, although it is implanted with ions. In addition, although not shown in the figure, a peripheral portion of the delaminated wafer located slightly inside the chamfered portion 8, i.e., a portion on which so-called polishing sag was produced, similarly is not bonded to the base wafer, and it may remain on the delaminated wafer 5 even after the delamination.
If the surface of the delaminated wafer 5 produced as a by-product as described above is mirror-polished in order to reclaim the delaminated wafer as a mirror surface wafer, the step 10 and surface roughness on the surface of delaminated wafer 5 are eliminated.
However, in the delaminated wafer 5 polished as described above, a part of the ion-implanted layer 9 on the chamfered portion 8 remains as shown in FIG. 4E. It was found that, if this wafer 5 was subjected to a heat treatment such as thermal oxidation treatment, the ion-implanted layer 9 remaining on the chamfered portion 8 was delaminated during the heat treatment process as shown in FIG. 4F, and the delaminated ion-implanted layer become particles 13, which adhere to the wafer. Furthermore, it was clarified by the inventors of the present invention that, because it was difficult to remove the particles adhered during such a heat treatment even with subsequent cleaning, there arose a problem that they degraded quality, yield and so forth of reclaimed wafers. Such a problem arises not only in the case of using a silicon wafer, but also in the reclaim treatment for cases of using a wafer of SiO2, SiC, AL2O3 etc. as a wafer to be implanted with ions.
Therefore, an object of the present invention is to provide a method for reclaiming a delaminated wafer, which provides a reclaimed wafer of high quality that does not generate particles even when it is subjected to a heat treatment with good yield, and such a reclaimed wafer.
According to the present invention, in order to achieve the aforementioned object, there is provided a method for reclaiming a delaminated wafer produced as a by-product in the production of bonded wafer by the ion implantation and delamination method, characterized in that at least an ion-implanted layer on a chamfered portion of the delaminated wafer is removed, and then a surface of the wafer is polished.
If at least the part of ion-implanted layer on the chamfered portion of the delaminated wafer is surely removed, and then the wafer surface is polished as described above, the ion-implanted layer is completely removed from the whole delaminated wafer. Therefore, particles are not generated even if the wafer is subsequently subjected to a heat treatment, and thus the wafer can be surely reclaimed as a wafer of high quality with high yield.
According to the present invention, there is also provided a method for reclaiming a delaminated wafer produced as a by-product in the production of bonded wafer by the ion implantation and delamination method, characterized in that at least a chamfered portion of the delaminated wafer is subjected to an etching treatment and/or processing by chamfering, and then a surface of the wafer is polished.
If at least the chamfered portion of the delaminated wafer is subjected to an etching treatment and/or processing by chamfering before reclaim polishing, the ion-implanted layer on the chamfered portion of the delaminated wafer can be removed. Then, by the subsequent polishing, the ion-implanted layer remained around the peripheral portion inside the chamfered portion is removed, and removal of the damage layer on the surface of the delaminated wafer and improvement of surface roughness can also be simultaneously attained. Therefore, the ion-implanted layer does not remain after the polishing, and hence, even if the wafer is subsequently subjected to a heat treatment, particles are not generated, and thus the wafer can be reclaimed as a wafer of high quality with high yield.
In this case, at least the ion-implanted layer on the chamfered portion of the delaminated wafer is preferably removed by subjecting at least the chamfered portion to the aforementioned etching treatment and/or processing by chamfering.
By removing at least the portion of the ion-implanted layer on the chamfered portion as described above, the ion-implanted layer, which has been a cause of the generation of particles, can surely be removed, and a reclaimed wafer not generating particles can be surely obtained by performing polishing thereafter.
According to the present invention, there is also provided a method for reclaiming a delaminated wafer produced as a by-product in the production of bonded wafer by the ion implantation and delamination method, characterized in that the delaminated wafer is subjected to a heat treatment, and then a surface of the wafer is polished.
By subjecting a delaminated wafer to a heat treatment beforehand as described above, the ion-implanted layer remaining around the peripheral portion of the delaminated wafer can be delaminated and it can be removed by polishing the wafer after cleaning or directly after the heat treatment. Further, surface roughness can be improved by the polishing, and thus a wafer of high quality can be reclaimed with good yield.
According to the present invention, there is further provided a wafer reclaimed by any one of the aforementioned methods.
In a wafer reclaimed as described above, all of the ion-implanted layer is removed, the damage layer on the surface of the delaminated wafer is removed, and surface roughness is also improved. Therefore, even if the wafer is subjected to a heat treatment thereafter, particles are not generated, and thus the wafer can be preferably used as a mirror surface wafer of high quality.
In particular, a delaminated wafer produced from a CZ silicon wafer as a by-product is preferably used as a base wafer or a usual silicon mirror surface wafer, because oxygen precipitation is caused in the reclaimed delaminated wafer by delamination heat treatment or the like, and it exerts the gettering effect.
Further, a delaminated wafer produced from an FZ silicon wafer as a by-product or a delaminated wafer having an epitaxial layer is preferred for reuse as a bond wafer, because such wafers do not contain crystal defects such as COPs (Crystal Originated Particles) and oxide precipitates like a CZ silicon wafer.
As explained above, since the ion-implanted layer is surely removed also for the chamfered portion in a delaminated wafer reclaimed according to the present invention, even if it is subjected to a heat treatment, particles resulting from the ion-implanted layer are not generated, and removal of the damage layer on the surface of the delaminated wafer and improvement of the surface roughness are also attained. Therefore, a wafer of high quality can be reclaimed with good yield.